The spinal column of humans provides support for the body and protection to the delicate spinal cord and nerves. The spinal column comprises a series of vertebrae stacked on top of each other. Each vertebra has a relatively large vertebral body located in the anterior portion of the spine and provides the majority of the weight-bearing support of the vertebral column. Each vertebral body has relatively strong bone comprising the outside surface of the body (cortical) and relatively weak bone comprising the center of the body (cancellous). Situated between each vertebral body is an intervertebral disc, which provides for cushioning and dampening of compressive forces to the spinal column. Located just posterior to the vertebral body and intervertebral disc is the spinal canal containing the delicate spinal cord and nerves. Posterior to the spinal canal are the different articulating processes of the vertebrae.
Various types of spinal column disorders are known and include scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal forward curvature of the spine, usually in the thoracic spine), excess lordosis (abnormal backward curvature of the spine, usually in the lumbar spine), spondylolisthesis (forward displacement of one vertebra over another, usually in a lumbar or cervical spine) and other disorders caused by abnormalities, disease, or trauma, such as ruptured or slipped discs, degenerative disc disease, fractured vertebra, tumors, and the like. Patients that suffer from such conditions usually experience extreme and debilitating pain and often neurologic deficit.
Spinal fusion is a technique often utilizing surgical implants which mechanically immobilize areas of the spine with eventual incorporation of grafting material. Such techniques have been used effectively to treat the above-described conditions and, in most cases, to relieve pain suffered by the patient. However, there are some disadvantages to the present fixation devices.
One technique for spinal fixation includes immobilization of the spine by the use of spine rods that run generally parallel to the spine. In practicing this technique, the posterior surface of the spine is exposed, and bone screws are first fastened to the pedicles of the appropriate vertebrae or to the sacrum, acting as anchor points for the spine rods. The bone screws are generally placed two per vertebrae, one at each pedicle on either side of the spinous process. Clamp assemblies join the spine rods to the screws. The spine rods are generally bent to achieve the desired curvature of the spinal column. Such systems are very stable but require implanting screws into each vertebrae to be treated. Also, since the pedicles of vertebrae above the second lumbar vertebra (L-2) are very small, only small bone screws can be used, which sometimes do not give the needed support to stabilize the spine. To stabilize the unstable spine sufficiently, one to two vertebrae above and one to two vertebrae below the area to be treated are often used for implanting the screws. The rods and clamps are surgically fixed to the spine from a posterior approach.
Anterior fixation devices have also been used, such as anterior plate systems. One type of anterior plate system involves a titanium plate with unicortical titanium bone screws that may lock to the plate and are placed over the anterior surface of the vertebral body. Another type of anterolateral plate system uses bicortical screws that may not lock to the plate. The bone screws have to be long enough to bite into both sides of the vertebral body to gain enough strength to obtain the needed stability. These devices are difficult to place due to the length of the screws, and damage occurs when the screws are misplaced.
A third type of anterior fixation device comprises a hollow device that may or may not be externally threaded. The device is positioned between two adjacent vertebral bodies. Bone grafts from cadavers or from the pelvic region of the patient may be placed into the hollow center of the device. Bone morphogenic protein or other substances that promote bone growth can also be placed into the hollow center of the device. The cage might allow bone to grow through the device and fuse the two adjacent vertebrae.
Although the devices described above present various solutions, further improvement in this area is desirable. In particular, improved stabilization devices that enable a surgeon to reduce a vertebral displacement (e.g., of the type occurring in spondylolisthesis) prior to spinal fixation are needed.